Coker furnaces typically have four to six tube passes per pair of drums, often configured with two passes per furnace cell. It is not usual for one or more of these to “coke up”—to become constricted due to coke formation with the furnace cell—during use. Removal of this coke buildup is referred to as “decoking.” Decoking may be necessary annually or more often, depending on various factors including design of the coking unit and the nature of the feed.
Historically, for decoking of a furnace cell, the associated (often two) delayed coking drums and the associated furnaces were fully shut down, after which the furnace cell was steam air decoked or offline decoked by mechanically removing the coke with scrapers driven through in the tubes by water (typically referred to as ‘pigging decoking’).
More recently, larger cokers have been built with two or more furnaces operating in parallel with multiple coke drums and with each furnace having multiple tube passes. Safe on-line pigging, that is, pigging decoking of a furnace cell while the associated delaying coking drums have continued to operate from input from other furnace cells, has not been available because, among other reasons, the piping system configuration must facilitate hazardous energy to be adequately contained in each step of operation.
A process referred to as “on-line spalling” has been used to decoke feed in a delayed coking system. In “on-line spalling,” steam is used in typically two of the passes to displace the feed and subsequently to raise the tube metal temperature until the coke spalls off the tube. Decoking effluent is then escorted by the steam to mix with the process furnace effluent from other on-line passes and the combined stream flows to the coke drum. This beneficially permits decoking without shutting a unit down.
More particularly, on-line spalling occurs when steam is introduced through the two furnace coils displacing the process fluid. The steam flow, upon leaving the furnace, rejoins the process fluid furnace effluent from the passes which remain in service. Then, the temperature of the steam and the tube passes which are to be decoked are raised. This temperature increase continues until reaching a temperature far in excess of normal operations. This may be as high as 1250 F depending on the tube material, thickness. However, piping from furnace outlets typically predates the advent of on-line spalling. As a result, existing piping and valve systems lack sufficient robustness to permit the higher temperatures of on-line spalling. Attempts to on-line spall with this existing equipment may result in failure as the heated piping and components fail pressure containment. Alternatively, that piping may be unusable for generation of a useful pipe stress analysis to determine whether on-line spalling is possible. For example, use of conventional low alloy materials, such as 9Cr-1Mo, would result in operation in the material's creep range, together with the difficulty in pipe stress analysis. Thus, for on-line spalling, a higher alloy is typically required to meet regulatory requirements.
Additionally, on-line pigging decoking of these furnace tubes would provide benefits. While some feeds can be successfully on-line spalled several times in a row, because of inorganic deposition or similar reason, the tubes may not return fully to original start-of-run temperatures. Eventually, these coker furnaces need to be cleaned by mechanical scraper (pigging) decoking. As a result, the need has been recognized for both on-line spalling and on-line mechanical scraper cleaning. On-line pigging decoking presents operational and structural issues, such as safe transitioning among process steps and ensuring components can provide the safety factors needed for on-line pigging.
While both safe on-line pigging of the furnace and on-line spalling of the main line may be attractive to the operator of a delayed coking system, safe on-line pigging in conjunction with safe on-line spalling presents additional issues. In particular, the valves and lines used to provide a system for safe on-line pigging process, must also be able to sustain the temperatures and pressure line conditions of on-line spalling.
No piping configuration has been identified which permits safe hazardous energy isolation through all steps for on-line pigging. Nor has a piping configuration been identified which is competent to sustain the temperatures and pressure line conditions of on-line spalling.